A transmission device designed as an 8-gear multi-stage transmission with frictional shifting elements such as disk clutches and disk brakes is known from DE 10 2005 002 337 A1. When a shift is demanded for a gear ratio change in the transmission device, at least one frictional shifting element, which is engaged in the force flow of the transmission device to obtain the actual gear currently engaged in the transmission device, has to be disengaged from the force flow of the transmission device, whereas at least one other frictional shifting element, which is disengaged from the force flow of the transmission device while the current, actual gear is engaged in the transmission device, has to be engaged in the force flow of the transmission device in order to obtain the target gear required.
During this, with increasing shift duration the torque transmitted by the frictional shifting element engaged in the force flow in order to obtain the gear currently engaged in the transmission device is transferred to a greater or lesser extent to the frictional shifting element that has to be engaged in the force flow of the transmission device in order to obtain the target gear required, whereas the torque that can be transmitted by the shifting element to be disengaged decreases.
Disadvantageously, frictional shifting elements in the open operating condition generate drag torques which impair the overall efficiency of an automatic transmission to an undesired extent.
For that reason transmission devices such as that known from DE 10 2008 000 429 A1 are increasingly often made, besides frictional shifting elements, also with interlocking shifting elements in the area of which no drag torques that impair the overall efficiency of a transmission device occur.
In that case, however, it must be borne in mind that only when they are close to their synchronous point can interlocking shifting elements be shifted from an open operating condition in which no torque can be transmitted by the interlocking shifting element, to their closed operating condition in which all of the torque applied can be transmitted by the interlocking shifting element. Furthermore, interlocking shifting elements engaged in the force flow of a transmission device can only be disengaged from the force flow with small shifting forces when they are close to their load-free operating condition.
When a command is given to close an interlocking shifting element by acting upon it with an appropriate closing force, then on the part of an electric control unit or by software means, the shift command is issued with a certain lead time before the synchronous point of the interlocking shifting element has been reached, which the interlocking shifting element must be at when the target gear is engaged. This is intended to ensure that the actual engagement process or interlocking of the interlocking shifting element takes place within a predetermined speed difference window between the two halves of the shifting element, within which the probability is high that the interlocking shifting element can be engaged. In addition, the lead time allows for system-inherent delays caused for example by signal propagation times, hydraulic reaction delays and/or the component travel path to be covered in order to establish the connection. In this context the lead time to be allowed, particularly in the case of hydraulically actuated interlocking shifting elements of transmission devices such as automatic transmissions, depends to an appreciable extent also on the operating temperature of the transmission device or the temperature of the transmission oil.
The lead time to be allowed in each case for initiating the closing process of the interlocking shifting element can for example be determined empirically. Moreover, the lead time can also be determined as a function of the currently existing gradient of the rotational speed variation of a transmission input shaft or a rotational speed equivalent thereto, such as a turbine rotational speed or the like, before reaching the synchronous point of the interlocking shifting element that determines the time when the interlocking shifting element can be engaged, and the actuation of the interlocking shifting element can be specified accordingly.
For the calculation of gradients of rotational speed variations in most cases several rotational speed values are used because of signal noise, in order to avoid erroneous control commands. However, as a result of this procedure it is possible, for example, that substantial rotational speed changes cannot be recognized promptly by virtue of gradients determined in this manner, since the complex calculation methods used for the avoidance of errors in each case delay the perception of changes.
If the closing process of an interlocking shifting element is not started in good time before the synchronous point of the interlocking shifting element is reached, the attempt to close the interlocking shifting element takes place while the rotational speed difference between the two halves of the interlocking shifting element is still too large. The interlocking shifting element cannot then be changed to its closed operating condition and it is also possible that damage may occur in the area of the interlocking shifting element if the shifting element halves to be brought into interlocked engagement with one another in fact only make contact in the area of their ends facing toward one another, and the abrasion that occurs during this can impair the function of the interlocking shifting element.